Techniques for selective enhancement of a digital image

Abstract

By automatically compensating for unwanted lighting effects caused by varying ambient light sources, digital still images and video are made to appear more naturally realistic. A set of correction factors based upon an ascertained ambient lighting conditions, such as color temperature, provides for selectively color correcting only those pixels requiring correction. In this way, only those portions of a digital image having excessive color due to ambient light conditions are improved without affecting those portions of the digital image not requiring correction.

What is claimed is:  
     
       1. A method of automatically color correcting a digital image taken under an ambient light source, comprising: 
       (a) converting the digital image to a selected color space, wherein each of the pixels has an associated set of color space components;  
       b) determining a characteristic color temperature of the ambient light source;  
       (c) selecting a first color component and a second color component of the set of color space components having a first color component value and a second color component value, respectively;  
       (d) determining the number of pixels of the digital image having both their respective first color component value within a predetermined range of first color component values and their second color component values within a predetermined range of second color component values, wherein the range of first color component values and the range of second color component values are based upon the characteristic ambient color temperature; and  
       (e) if the number of pixels from (d) is greater than a pixel threshold value, then for each pixel in the digital image having its respective first color component value within the predetermined range and its second color component value within the predetermined range of second color component values, modifying that pixel's set of color components.  
     
     
       2. A method as recited in  claim 1 , wherein the selected color space is a hue-based color space selected from the group comprising: Hue, Saturation, and Value (HSV) color space, Hue, Saturation, Intensity (HSI) color space, Hue, Light, Saturation (HLS) color space, Hue, Saturation, and Brightness (HSB), and Tek HVC (Hue, Value, and Chroma). 
     
     
       3. A method as recited in  claim 2 , wherein the selected color space is the HLS color space wherein the set of color space components includes a hue component, a lightness component, and a saturation component, wherein the first color component is the hue component and the second color component is the saturation component. 
     
     
       4. A method as recited in  claim 3 , wherein the determining (b) comprises: 
       determining a predominant frequency of ambient light; and  
       assigning the ambient color temperature based upon the determined predominant frequency.  
     
     
       5. A method as recited in  claim 3 , wherein the determining (b) comprises: 
       manually inputting a selected ambient light source; and  
       assigning the ambient color temperature based upon the selected ambient light source.  
     
     
       6. A method as recited in  claim 5 , wherein the assigning is based upon a look up table that cross relates the selected ambient light source with the ambient color temperature. 
     
     
       7. A method as recited in  claim 4  or  6 , wherein the predetermined range of hue component values is based upon a ramp function. 
     
     
       8. A method as recited in  claim 7 , wherein the ramp function is formed of a first portion contiguous with a second portion which, in turn, is contiguous with a third portion. 
     
     
       9. A method as recited in  claim 8 , wherein the first portion is a monotonically increasing linear function having a first portion minimum value at a first hue component H 1  and a first portion maximum value at a second hue component H 2  and wherein the second portion is a constant value being the maximum value, and wherein the third portion is a monotonically decreasing linear function having a third portion maximum value at the hue component H 3  being the maximum value and a third portion minimum value at a hue component H 4 . 
     
     
       10. A method as recited in  claim 9 , wherein the predetermined range of hue values is the range of hue components between approximately H 1  and H 4 . 
     
     
       11. A method as recited in  claim 8 , wherein the first portion and the third portions of the ramp function are a cubic parabolic function having the hue component as an independent variable. 
     
     
       12. A method as recited in  claim 7 , wherein the selected ambient light source is selected from the group consisting of an incandescent light source and a fluorescent light source. 
     
     
       13. An apparatus for enhancing a digital image made up of pixels by removing a particular color, comprising: 
       a first means for converting the digital image to a selected color space, wherein each of the pixels has an associated set of color space components;  
       a second means coupled to the first means for determining a characteristic color temperature of the ambient light source;  
       a third means coupled to the second means for selecting a first color component and a second color component of the set of color space components having a first color component value and a second color component value, respectively;  
       a fourth means coupled to the third means for determining the number of pixels of the digital image having both their respective first color component value within a predetermined range of first color component values and their second color component values within a predetermined range of second color component values, wherein the range of first color component values and the range of second color component values are based upon the characteristic ambient color temperature; and  
       a fifth means for modifying that pixel's set of color components when the number of pixels from determined by the fourth means is greater than a pixel threshold value, then for each pixel in the digital image having its respective first color component value within the predetermined range and its second color component value within the predetermined range of second color component values.